This invention relates to the preparation of selenium for incorporation into a glass batch.
Selenium is an important additive for making various heat absorbing and colored glass. It has been used in the glass industry for more than 100 years to produce colors from pink to red to blue and even as a de-colorizing agent by compensating for the blue green color resulting from the incorporation of ferrous (Fe.sup.2+) material. Its desirable coloring properties and long history of wide spread use, however, do not speak of the difficulties involved in its utilization.
Selenium is considered one of the most expensive components of a soda lime silica glass batch. Fortunately, only parts per million (PPM) levels are needed for most coloring needs. However, concerns about selenium's extreme volatility outweigh the small amounts required for coloration.
The volatilization of selenium and its compounds are of important technological significance to glass makers. According to conventional practice, in most cases the selenium, in the form of metal or selenium compounds, is mixed and melted with other total amounts of the raw materials for the glass. Selenium losses of nearly 85% are common in glass production facilities. Selenium is so thermally unstable that volatilization for the metallic state begins at about 200.degree. C. (which is below its melting point of 217.degree. C.). The economic and corresponding environmental costs of selenium volatility are therefore of great concern.
There are only two forms of selenium which produce color in soda lime silica glass, namely, selenium in its elemental state (Se.sup.o) and polyselenide state (Se.sub.x.sup.2-). The neutral form creates a pink color whereas the polyselenide's contribution depends on the associated species, for example, FeSe produces red brown color. The other possible forms of selenium in the vitreous state Se.sup.2-, Se.sup.4+ and Se.sup.6+ are colorless. Glass redox, therefore, will have great influence on the final color of the glass. Clearly the problems associated with selenium go beyond just its retention in the melt and must address the efficient development of the correct color of the final glass product. As a starting point, however, selenium must be retained before dealing with the resulting color.
One method of retaining selenium is to combine the selenium with either glass cullet or a portion of the glass batch material and sinter the resulting mixture as disclosed in U.S. Pat. Nos. 3,291,585 and 3,628,932. The sintered material is then recombined with the glass batch and melted.
It would be advantageous to provide a system of adding selenium to a glass batch without the necessity of pre-reacting the selenium prior to adding it to the glass batch.